The current aims of this research program all involve electron microscopy of DNA and include: 1. The construction by DNA heteroduplex methods, of a physical/genetic map of the Salmonella phage P22 using insertions of translocatable drug-resistance elements at known genetic locations and deletions with knwon genetic endpoints (some of which are generated by the translocatabl elements) to make the correlation between physical distance and genetic location. 2. Application of similar methods to construct physical maps of segments of the Salmonella chromosome transposed onto F' plasmids. 3. The analysis of the mechanism of excision of translocatable drug-resistance elements by heteroduplex analysis of the DNA of drug-sensitive and deletion "revertants" of bacteriophages (coliphage lambda as well as P22) carrying the various elements inserted into their genomes.